Activity regulation and physiological impacts of maize C4-specific phosphoenolpyruvate carboxylase overproduced in transgenic rice plants

Phosphoenolpyruvate carboxylase (PEPC) was overproduced in the leaves of rice plants by introducing the intact maize C₄-specific PEPC gene. Maize PEPC in transgenic rice leaves underwent activity regulation through protein phosphorylation in a manner similar to endogenous rice PEPC but contrary to that occurring in maize leaves, being downregulated in the light and upregulated in the dark. Compared with untransformed rice, the level of the substrate for PEPC (phosphoenolpyruvate) was slightly lower and the product (oxaloacetate) was slightly higher in transgenic rice, suggesting that maize PEPC was functioning even though it remained dephosphorylated and less active in the light. ¹⁴CO₂ labeling experiments indicated that maize PEPC did not contribute significantly to the photosynthetic CO₂ fixation of transgenic rice plants. Rather, it slightly lowered the CO₂ assimilation rate. This effect was ascribable to the stimulation of respiration in the light, which was more marked at lower O₂ concentrations. It was concluded that overproduction of PEPC does not directly affect photosynthesis significantly but it suppresses photosynthesis indirectly by stimulating respiration in the light. We also found that while the steady-state stomatal aperture remained unaffected over a wide range of humidity, the stomatal opening under non-steady-state conditions was destabilized in transgenic rice. This revised version was published online in August 2006 with corrections to the Cover Date.     

Carbonic anhydrase activity in leaves and its role in the first step of c(4) photosynthesis

In C₄ plants carbonic anhydrase catalyzes the critical first step of C₄ photosynthesis, the hydration of CO₂ to bicarbonate. The maximum activity of this enzyme in C₄ leaf extracts, measured by H⁺ production with saturating CO₂ and extrapolated to 25°C, was found to be 3,000 to 10,000 times the maximum photosynthesis rate for these leaves. Similar activities were found in C₃ leaf extracts. However, the calculated effective activity of this enzyme at in vivo CO₂ concentrations was apparently just sufficient to prevent the rate of conversion of CO₂ to HCO₃⁻ from limiting C₄ photosynthesis. This conclusion was supported by the mass spectrometric determination of leaf carbonic anhydrase activities.     

Carbon isotope fractionation in plants

Plants with the C₃, C₄, and crassulacean acid metabolism (CAM) photosynthetic pathways show characteristically different discriminations against ¹³C during photosynthesis. For each photosynthetic type, no more than slight variations are observed within or among species. CAM plants show large variations in isotope fractionation with temperature, but other plants do not. Different plant organs, subcellular fractions and metabolises can show widely varying isotopic compositions. The isotopic composition of respired carbon is often different from that of plant carbon, but it is not currently possible to describe this effect in detail. The principal components which will affect the overall isotope discrimination during photosynthesis are diffusion of CO₂, interconversion of CO₂ and HCO^?₃, incorporation of CO₂ by phosphoenolpyruvate carboxylase or ribulose bisphosphate carboxylase, and respiration. Theisotope fractionations associated with these processes are summarized. Mathematical models are presented which permit prediction of the overall isotope discrimination in terms of these components. These models also permit a correlation of isotope fractionations with internal CO₂ concentrations. Analysis of existing data in terms of these models reveals that CO₂ incorporation in C₃ plants is limited principally by ribulose bisphosphate carboxylase, but CO₂ diffusion also contributes. In C₄ plants, carbon fixation is principally limited by the rate of CO₂ diffusion into the leaf. There is probably a small fractionation in C₄ plants due to ribulose bisphosphate carboxylase.     

Photosynthetic and Respiratory Activity of Fruiting Forms within the Cotton Canopy

The supply of photosynthates by leaves for reproductive development in cotton (Gossypium hirsutum L.) has been extensively studied. However, the contribution of assimilates derived from the fruiting forms themselves is inconclusive. Field experiments were conducted to document the photosynthetic and respiratory activity of cotton leaves, bracts, and capsule walls from anthesis to fruit maturity. Bracts achieved peak photosynthetic rates of 2.1 micromoles per square meter per second compared with 16.5 micromoles per square meter per second for the subtending leaf. However, unlike the subtending leaf, the bracts did not show a dramatic decline in photosynthesis with increased age, nor was their photosynthesis as sensitive as leaves to low light and water-deficit stress. The capsule wall was only a minor site of ¹⁴CO₂ fixation from the ambient atmosphere. Dark respiration by the developing fruit averaged −18.7 micromoles per square meter per second for 6 days after anthesis and declined to −2.7 micromoles per square meter per second after 40 days. Respiratory loss of CO₂ was maximal at −158 micromoles CO₂ per fruit per hour at 20 days anthesis. Diurnal patterns of dark respiration for the fruit were age dependent and closely correlated with stomatal conductance of the capsule wall. Stomata on the capsule wall of young fruit were functional, but lost this capacity with increasing age. Labeled ¹⁴CO₂ injected into the fruit interior was rapidly assimilated by the capsule wall in the light but not in the dark, while fiber and seed together fixed significant amounts of ¹⁴CO₂ in both the light and dark. These data suggest that cotton fruiting forms, although sites of significant respiratory CO₂ loss, do serve a vital role in the recycling of internal CO₂ and therein, function as important sources of assimilate for reproductive development.     

Phosphoenolpyruvate Carboxylase and CarbonEconomy of Apple Seedlings

Seeds of apple cv. Golden Delicious were germinated and cultivatedin the greenhouse until the third leaf emerged. Respirationofgerminating seeds or photosynthesis of the first leaves wasmeasured by infra-red gas analysis and porometry, respectively.To study the role of phosphoenolpyruvate carboxylase (PEPC),the dominant carboxylase in the carbon economy, its CO₂ refixationpotentialwas related to the amount of CO₂ lost in respiration. With arange of 0.2 (dry seeds) to 18 (cotyledons) µmol CO₂ h^–1g^–1 PEPC activity resembled or exceeded the amount ofC0₂ lost in respiration before the third leaf developed. Itis concludedthat PEPC largely contributes to economize the carbonmetabolism of apple seedlings before they become photosyntheticallycompetent. Key words: Apple (Malus pumila Mill.) seedling, carbon economy, phosphoenolpyruvate carboxylase, photosynthesis, respiration     

Growth and physiological responses of cotton (Gossypium hirsutum L.) to elevated carbon dioxide and ultraviolet-B radiation under controlled environmental conditions

Better understanding of crop responses to projected changes in climate is an important requirement. An experiment was conducted in sunlit, controlled environment chambers known as soil–plant–atmosphere–research units to determine the interactive effects of atmospheric carbon dioxide concentration [CO₂] and ultraviolet‐B (UV‐B) radiation on cotton (Gossypium hirsutum L.) growth, development and leaf photosynthetic characteristics. Six treatments were used, comprising two levels of [CO₂] (360 and 720 µmol mol^?1) and three levels of 0 (control), 7.7 and 15.1 kJ m^?2 d^?1 biologically effective UV‐B radiations within each CO₂ level. Treatments were imposed for 66 d from emergence until 3 weeks after the first flower stage. Plants grown in elevated [CO₂] had greater leaf area and higher leaf photosynthesis, non‐structural carbohydrates, and total biomass than plants in ambient [CO₂]. Neither dry matter partitioning among plant organs nor pigment concentrations was affected by elevated [CO₂]. On the other hand, high UV‐B (15.1 kJ m^?2 d^?1) radiation treatment altered growth resulting in shorter stem and branch lengths and smaller leaf area. Shorter plants at high UV‐B radiation were related to internode lengths rather than the number of mainstem nodes. Fruit dry matter accumulation was most sensitive to UV‐B radiation due to fruit abscission. Even under 7.7 kJ m^?2 d^?1 of UV‐B radiation, fruit dry weight was significantly lower than the control although total biomass and leaf photosynthesis did not differ from the control. The UV‐B radiation of 15.1 kJ m^?2 d^?1 reduced both total (43%) and fruit (88%) dry weights due to smaller leaf area and lower leaf net photosynthesis. Elevated [CO₂] did not ameliorate the adverse effects of UV‐B radiation on cotton growth and physiology, particularly the boll retention under UV‐B stress.     

Temporal expression of effects of varying nitrogen supply on canopy growth, photosynthesis and fruit production for Actinidia deliciosa vines in the field

The effects of varying nitrogen supply on canopy leaf area, response of leaf net photosynthesis (A_n) to quantum flux density (Q), and fruit yields of kiwifruit vines (Actinidia deliciosa var. deliciosa) were examined in a two-year field experiment. Vines were grown with 0, 250 or 750 kg N ha^?1 year^?1. The responses to nitrogen supply were compared with responses to shade, to examine the impact of reduced carbon assimilation on canopy leaf area and fruit yields. Nitrogen supply did not affect significantly any of the measured variables during the first season of the experiment. In the second season, canopy leaf area was reduced significantly where nitrogen supply was limited. The quantum efficiency of photosynthesis (φ_q) increased from 0. 03 mol CO₂ mol^?1 Q soon after leaf emergence to more than 0. 05 mol CO₂ mol^?1 Q during the middle of the growing season. The quantum saturated rate of A_n (A_sat) also increased during the season, from 7–10 μmol CO₂ m^?2 s^?1 soon after leaf emergence, to 15–20 (μmol CO₂ m^?2 s^?1 during the middle of the growing season. φ_q and A_sat increased significantly with nitrogen supply at all measurement times during the second season. For vines with high nitrogen, fruit yields in both seasons were similar, averaging 3. 05 kg m^?2. Fruit yields in the second season were reduced significantly where nitrogen supply was limited, due to reduced fruit numbers. The relative effects of reduced leaf area and reduced leaf photosynthesis for carbon assimilation by nitrogen deficient vines were examined using a mathematical model of canopy photosynthesis for kiwifruit vines. Simulations of canopy photosynthesis indicated that effects on leaf area and on leaf photosynthesis were of similar importance in the overall effects of nitrogen deficiency on carbon assimilation. The effects of nitrogen supply on fruit numbers (i. e. flower development) preceded the measured effects on carbon assimilation, indicating that the nitrogen supply affected carbon partitioning to reserves in the first season.     

Photosynthetic and respiratory characterization of field grown tomato

The photosynthetic responses of tomato (Lycopersicum esculentum Mill.) leaves to environmental and ontogenetic factors were determined on plants grown in the field under high radiation and high nitrogen fertilization. Response curves showed net photosynthesis to only approach light saturation at a photosynthetic photon flux density (PPFD) of 2200 mol m^-2 s^-1, with rates of approx. 40 mol CO₂ m^-2 s^-1. A broad temperature optimum was observed between 25° and 35°C, with 50% of the photosynthetic rates remaining even at 47°C. The high rate, the lack of saturation at the equivalent of full sunlight, and the tolerance to high temperature of tomato were unusual in light of the literature on this C₃ species. Apparently, acclimation to the field environment of high radiation and hot daytime temperature, coupled with the high nitrogen nutrition, made possible the high photosynthetic performance normally associated with C₄ species.Photosynthetic ability of the leaf reached a maximum near the time of its full expansion and declined steadily thereafter, regardless of the time of leaf initiation. Leaf nitrogen content showed a similar decline with leaf ontogeny. Photosynthesis was linearly correlated with nitrogen content, whether the nitrogen variation was due to leaf age or rates of nitrogen fertilization. Internal CO₂ concentrations (C_i) of the leaf indicated that stomatal function was well coordinated with photosynthetic capacity as leaf age and fluence rate varied down to a PPFD of 500 mol m^-2 s^-1. As PPFD decreased further, there was less stomatal control and C_i increased to as high as 320 bar bar^-1.Dark respiration was highest for expanding leaves and increased nearly exponentially with temperature. Respiration was also highest for young and expanding fruits, and next highest for fruits just turning pink. Fruit respiration increased approximately linearly with temperature, and was estimated to be an important component of the CO₂ flux of the plant near maturity because of the heavy fruit load and low leaf photosynthesis at that time. The results are significant for model simulation of tomato productivity in the field.     

Effect of fruiting on carbon budgets of apple tree canopies

Summary Carbon budgets were calculated from net photosynthesis and dark respiration measurements for canopies of field-grown, 3-year-old apple trees (Malus domestica Borkh.) with maximum leaf areas of 5.4 m² in a temperature-controlled Perspex tree chamber, measured in situ over 2 years (July 1988 to October 1990) by computerized infrared gas analysis using a dedicated interface and software. Net photosynthesis (Pn) and carbon assimilation per leaf area peaked at respectively 8.3 and 7.7 mol CO₂ m^–2 s^–1 in April. Net photosynthesis (Pn) and dark respiration (Rd) per tree peaked at 3.6 g CO₂ tree^–1 h^–1 (Pn) and 1.2 g CO₂ tree^–1 h^–1 (Rd), equivalent to 4.2 mol CO₂ (Pn) and 1.4 mol CO₂ (Rd) m^–2 s^–1 with maximum carbon gain per tree in August and maximum dark respiration per tree in October 1988 and 1989. In May 1990, a tree was deblossomed. Pn (per tree) of the fruiting apple tree canopy exceeded that of the non-fruiting tree by 2–2.5 fold from June to August 1990, attributed to reduced photorespiration (RI), and resulting in a 2-fold carbon gain of the fruiting over the non-fruiting tree. Dark respiration of the fruiting tree canopy progressively exceeded, with increasing sink strength of the fruit, by 51% (June–August), 1.4-fold (September) and 2-fold (October) that of the non-fruiting tree due to leaf (i. e. not fruit) respiration to provide energy (a) to produce and maintain the fruit on the tree and (b) thereafter to facilitate the later carbohydrate translocation into the woody perennial parts of the tree. The fruiting tree reached its optium carbon budget 2–4 weeks earlier (August) then the non-fruiting tree (September 1990). In the winter, the trunk respired 2–100 g CO₂ month^–1 tree^–1. These data represent the first long-term examination of the effect of fruiting without fruit removal which shows increased dark respiration and with the increase progressing as the fruit developed.     

Photosynthesis of Ectocarpus siliculosus in red light and after pulses of blue light at high pH – evidence for bicarbonate uptake

Pulses of blue light cause stimulation of red light saturated photosynthesis in Ectocarpus siliculosus, because blue light activates the operation of a pathway for inorganic carbon (C_i) acquisition by inducing the mobilization of CO₂ from an intermediate metabolite. In the absence of exogenous C_i, photosynthetic rates roughly equal those of CO₂ release by respiration. In seawater of pH 9·5 (2·3 mol m^–3 total C_i, but concentrations of free CO₂ below 0·2 mmol m^–3), photosynthesis was clearly above these rates, although they were only ≈ 30% of those in normal seawater (≈ pH 8). The degree and the time course of the stimulations of photosynthesis by pulses of blue light were unaltered at high pH. Essentially the same characteristics were found after buffering or in the presence of acetazolamide, an inhibitor of extracellular carbonic anhydrase activity. Therefore, it is concluded that Ectocarpus is able to directly take up HCO₃^– in addition to CO₂ (uptake of CO₃^2– cannot be excluded). The dependence of photosynthesis on C_i at pH 9·5 was biphasic, with C_i below 0·2 mol m^–3 having no effect at all. In C_i-free seawater, the shapes of the stimulations after blue light pulses differed for pH 6, pH 8 and pH 9·5. At low pH, only the fast peak (maximum ≈ 5 min after blue light) was detected, whereas at high pH mainly the slow peak (maximum ≈ 20 min after blue light) was observed. At the intermediate pH 8, both peaks were present. As inhibition of total carbonic anhydrase by ethoxyzolamide brought out the fast peak of the stimulations at pH 9·5 it is concluded that the fast component was due to a transient disequilibrium of an intracellular pool of C_i which, after blue light, was fed by CO₂ released from the postulated storage intermediate.     

Photosynthetic and stomatal responses of the halophyte,Plantago maritima L. to fluctuations in salinity

Abstract Measurements of photosynthesis as a function of intercellular CO₂ (A-C₁ curve) were made on single. attached leaves of Plantago maritima L. while plants were exposed to changes in salinity. Salinity was increased in steps from 50 to 500 mol m^-3 NaCl and then returned to 50 mol m^-3 NaCl at two rates, 75 mol m^-3 (NaCl) day^-1 (experiment 1) and 150 mol m^-3 (NaCl) day^-1 (experiment 2). In experiment one, the CO₂ assimilation rate declined at high CO₂ concentrations, but the initial slope of the A-C₁ curve was unaffected in young leaves after salinity was increased to 500 mol m^-3 NaCl. The insensitivity of photosynthesis to increases in CO₂ concentration above air levels was not associated with insensitivity to a reduction in oxygen concentration. In experiment two increasing the rate at which salinity was changed resulted in larger declines in photosynthesis and leaf conductance than were observed in experiment one. Both the initial slope and the CO₂ saturated region of the A-C₁ curve were substantially reduced at high salinity suggesting that mesophyll biochemical capacity had been inhibited. However, concurrent measurements of photosynthesis as oxygen evolution under 5% CO₂ indicated no effect of increased salinity on photosynthetic capacity. This suggests that the apparent non-stomatal limitations indicated by A-C₁ measurements were artifacts caused by strong, nonuniform stomatal closure.     

A Comparison of Dark Respiration between C(3) and C(4) Plants

Lower respiratory costs were hypothesized as providing an additional benefit in C₄ plants compared to C₃ plants due to less investment in proteins in C₄ leaves. Therefore, photosynthesis and dark respiration of mature leaves were compared between a number of C₄ and C₃ species. Although photosynthetic rates were generally greater in C₄ when compared to C₃ species, no differences were found in dark respiration rates of individual leaves at either the beginning or after 16 h of the dark period. The effects of nitrogen on photosynthesis and respiration of individual leaves and whole plants were also investigated in two species that occupy similar habitats, Amaranthus retroflexus (C₄) and Chenopodium album (C₃). For mature leaves of both species, there was no relationship between leaf nitrogen and leaf respiration, with leaves of both species exhibiting a similar rate of decline after 16 h of darkness. In contrast, leaf photosynthesis increased with increasing leaf nitrogen in both species, with the C₄ species displaying a greater photosynthetic response to leaf nitrogen. For whole plants of both species grown at different nitrogen levels, there was a clear linear relationship between net CO₂ uptake and CO₂ efflux in the dark. The dependence of nightly CO₂ efflux on CO₂ uptake was similar for both species, although the response of CO₂ uptake to leaf nitrogen was much steeper in the C₄ species, Amaranthus retroflexus. Rates of growth and maintenance respiration by whole plants of both species were similar, with both species displaying higher rates at higher leaf nitrogen. There were no significant differences in leaf or whole plant maintenance respiration between species at any temperature between 18 and 42°C. The data suggest no obvious differences in respiratory costs in C₄ and C₃ plants.     

Atmospheric CO2 mole fraction affects stand‐scale carbon use efficiency of sunflower by stimulating respiration in light

Plant carbon‐use‐efficiency (CUE), a key parameter in carbon cycle and plant growth models, quantifies the fraction of fixed carbon that is converted into net primary production rather than respired. CUE has not been directly measured, partly because of the difficulty of measuring respiration in light. Here, we explore if CUE is affected by atmospheric CO₂. Sunflower stands were grown at low (200 μmol mol^?1) or high CO₂ (1000 μmol mol^?1) in controlled environment mesocosms. CUE of stands was measured by dynamic stand‐scale ¹³C labelling and partitioning of photosynthesis and respiration. At the same plant age, growth at high CO₂ (compared with low CO₂) led to 91% higher rates of apparent photosynthesis, 97% higher respiration in the dark, yet 143% higher respiration in light. Thus, CUE was significantly lower at high (0.65) than at low CO₂ (0.71). Compartmental analysis of isotopic tracer kinetics demonstrated a greater commitment of carbon reserves in stand‐scale respiratory metabolism at high CO₂. Two main processes contributed to the reduction of CUE at high CO₂: a reduced inhibition of leaf respiration by light and a diminished leaf mass ratio. This work highlights the relevance of measuring respiration in light and assessment of the CUE response to environment conditions.     

Utilization of inorganic carbon in the edible cyanobacterium Ge-Xian-Mi (Nostoc) and its role in alleviating photo-inhibition

The present work investigated the inorganic carbon (C_i) uptake, fluorescence quenching and photo‐inhibition of the edible cyanobacterium Ge‐Xian‐Mi (Nostoc) to obtain an insight into the role of CO₂ concentrating mechanism (CCM) operation in alleviating photo‐inhibition. Ge‐Xian‐Mi used HCO₃^– in addition to CO₂ for its photosynthesis and oxygen evolution was greater than the theoretical rates of CO₂ production derived from uncatalysed dehydration of HCO₃^–. Multiple transporters for CO₂ and HCO₃^– operated in air‐grown Ge‐Xian‐Mi. Na⁺‐dependent HCO₃^– transport was the primary mode of active C_i uptake and contributed 53–62% of net photosynthetic activity at 250 µmol L^?1 KHCO₃ and pH 8.0. However, the CO₂‐uptake systems and Na⁺‐independent HCO₃^– transport played minor roles in Ge‐Xian‐Mi and supported, respectively, 39 and 8% of net photosynthetic activity. The steady‐state fluorescence decreased and the photochemical quenching increased in response to the transport‐mediated accumulation of intracellular C_i. Inorganic carbon transport was a major factor in facilitating quenching during the initial stage and the initial rate of fluorescence quenching in the presence of iodoacetamide, an inhibitor of CO₂ fixation, was 88% of control. Both the initial rate and extent of fluorescence quenching increased with increasing external dissolved inorganic carbon (DIC) and saturated at higher than 200 µmol L^?1 HCO₃^–. The operation of the CCM in Ge‐Xian‐Mi served as a means of diminishing photodynamic damage by dissipating excess light energy and higher external DIC in the range of 100–10000 µmol L^?1 KHCO₃ was associated with more severe photo‐inhibition under strong irradiance.     

The effect of ammonium and nitrate on CO2 assimilation,RuBP and PEP carboxylase activity and dry matter production in wheat

Photosynthetic¹⁴CO₂ assimilation, ribulose 1, 5-bisphosphate carboxylase (RuBPC), phosphoenol pyruvate carboxylase (PEPC) and dry matter (DM) production were examined in wheat under varying levels and forms of nitrogen.¹⁴CO₂ assimilation increased gradually after germination reaching a peak value at anthesis, followed by a sharp decline. A similar pattern was observed for both the carboxylases, RuBPC and PEPC activities. Increase in nitrogen levels, in general, brought about a significant increase over the control (zero-nitrogen) in¹⁴CO₂ assimilation, RuBPC, PEPC activities and DM production. There were no significant differences in RuBPC activity and¹⁴CO₂ assimilation with respect to the forms of nitrogen. Significantly higher PEPC activity and DM was observed in plants supplied with nitrate-nitrogen (NO₃-N), as compared to those supplied with ammonium-nitrogen (NH₄-N). The significance of PEPC activity in C₃ photosynthesis is discussed in relation to DM distribution.     

Evidence that inducible C4-type photosynthesis is a chloroplastic CO2-concentrating mechanism in Hydrilla, a submersed monocot

Hydrilla verticillata (L.f.) Royle exhibits an inducible C₄-type photosynthetic cycle, but lacks Kranz anatomy. Leaves in the C₄-type state (but not C₃-type) contained up to 5-fold higher internal dissolved inorganic carbon (DIC) concentrations than the medium, indicating that they possessed a CO₂-concentrating mechanism (CCM). Several lines of evidence indicated that the chloroplast was the likely site of CO₂ generation. From C₄-type leaf [DIC] measurements, the estimated chloroplastic free [CO₂] was 400 mmol m^?3. This gave a calculated 2% O₂ inhibition of photosynthesis, which was identical to the measured value, and provided independent evidence that the estimated [CO₂] was close to the true value. A homogeneous distribution of DIC in the C₄-type leaf could not account for such a high [CO₂], or the resultant low O₂ inhibition. For C₃-type leaves the estimated chloroplastic [CO₂] was only 7 mmol m^?3, which gave high, and similar, calculated and measured O₂ inhibition values of 22 and 26%, respectively. The CCM did not appear to be located at the plasma membrane, as it operated at low and high pH, indicating that it was independent of use of HCO₃^? from the medium. Also, both C₃^? and C₄-type Hydrilla leaves showed pH polarity in the light, with abaxial and adaxial boundary layer values of about pH 4·0 and 10·5, respectively. Thus, pH polarity was not a direct component of the CCM, though it probably improved access to HCO₃. Additionally, iodoacetamide and methyl viologen greatly reduced abaxial acidification, but not the steady-state CCM. Inhibitor studies suggested that the CCM required photosynthetically generated ATP, but Calvin cycle activity was not essential. Both leaf types accumulated DIC in the dark by an ATP-requiring process, possibly respiration, and C₄-type leaves fixed CO₂ at 11·8% of the light rate. The operation of a CCM to minimize photorespiration, and the ability to recapture respiratory CO₂ at night, would conserve DIC in a densely vegetated lake environment where daytime [CO₂] is severely limiting, while [O₂] and temperatures are high.     

Atmospheric CO2 concentration does not directly affect leaf respiration in bean or poplar

It is a matter of debate if there is a direct (short‐term) effect of elevated atmospheric CO₂ concentration (C_a) on plant respiration in the dark. When C_a doubles, some authors found no (or only minor) changes in dark respiration, whereas most studies suggest a respiratory inhibition of 15–20%. The present study shows that the measurement artefacts – particularly leaks between leaf chamber gaskets and leaf surface, CO₂ memory and leakage effects of gas exchange systems as well as the water vapour (‘water dilution’) effect on DCO₂ measurement caused by transpiration – may result in larger errors than generally discussed. A gas exchange system that was used in three different ways – as a closed system in which C_a increased continuously from 200 to 4200 mmol (CO₂) mol^‐1 (air) due to respiration of the enclosed leaf; as an intermittently closed system that was repeatedly closed and opened during C_a periods of either 350 or 2000 mmol mol^‐1, and as an open system in which C_a varied between 350 and 2000 mmol mol^‐1– is described. In control experiments (with an empty leaf chamber), the respective system characteristics were evaluated carefully. When all relevant system parameters were taken into account, no effects of short‐term changes in CO₂ on dark CO₂ efflux of bean and poplar leaves were found, even when C_a increased to 4200 mmol mol^‐1. It is concluded that the leaf respiration of bean and poplar is not directly inhibited by elevated atmospheric CO₂.     

Carbonate ions appear to neither inhibit nor stimulate use of bicarbonate ions in photosynthesis by Ulva lactuca

Rates of photosynthesis by the marine macroalga Ulva lactuca were measured in a factorial experiment at five concentrations of HCO₃^? and CO₃^2- between 0·20 and 1·26 mol m^?3, but very low concentrations of CO₂. The results demonstrated that HCO₃^? was available for use, but an analysis of variance showed that CO₃^2- had neither an inhibiting nor a stimulating effect on rates of photosynthesis over this concentration range. Over the experiment, pH varied from 8·46 to 10·06 and this also had no significant effect on rates of photosynthesis. The lack of a stimulatory effect of high concentrations of CO₃^2- on the rate of photosynthesis at low concentrations of HCO₃^? was taken as circumstantial evidence for direct uptake of HCO₃^? rather than proton extrusion and external production of CO₂. In the rockpools in which U. lactuca grows, pH values up to 10·35 have been recorded, and for much of the time, CO₃^2- was the major form of inorganic carbon available. The apparent lack of an ability to use CO₃^2- under these conditions suggests that direct use of CO₃^2- as a source of inorganic carbon for photosynthesis is unlikely to be widespread.     

PHOTOSYNTHETIC CHARACTERISTICS OF C3-C4 INTERMEDIATE FLAVERIA FLORIDANA (ASTERACEAE) IN NATURAL HABITATS: EVIDENCE OF ADVANTAGES TO C3-C4 PHOTOSYNTHESIS AT HIGH LEAF TEMPERATURES

Measurements of leaf gas exchange were conducted in situ for the C₃-C₄ intermediate plant Flaveria floridana. Leaves exhibited measurable CO₂ assimilation at atmospheric CO₂ concentrations as low as 20 μmol/mol. This result demonstrates that the low CO₂ compensation points observed in past studies of greenhouse-grown C₃-C₄ intermediate plants also exist in plants growing in their natural habitat. Photosynthesis rates in F. floridana were near their maximum at intercellular CO₂ concentrations as low as 112 μmol/mol. The existence of near-maximum photosynthesis rates at such low intercellular CO₂ concentrations is interpreted as evidence for the existence of a CO₂-concentrating mechanism in F. floridana. Such a mechanism would also explain the observed lack of response in photosynthesis rates to reductions in stomatal conductance and intercellular CO₂ concentration as the leaf-to-air water vapor concentration gradient is increased. Photosynthetic rates were relatively high at leaf temperatures between 35 and 40 C, compared to most C₃ plants. At midday during May, when leaf temperatures were between 35 and 42 C, F. floridana leaves exhibited photosynthesis rates that were four times higher than a sympatric C₃ species (Eustoma exaltatum) of similar growth form and ecological habit. The high photosynthesis rates at high leaf temperatures in F. floridana were not due to higher leaf nitrogen contents, but rather to its reduced rate of photorespiration. These results confirm that C₃-C₄ intermediate photosynthesis can provide plants with an advantage at high leaf temperatures, compared to C₃ plants.